Systems having optical sensor arrays directly incorporated into a thin film transistor (TFT) liquid crystal display (LCD) have been proposed. Many different uses for such systems exist, for example, as a scanner, fingerprint sensor or touch screen. Such a system has two images: the image displayed on the LCD display screen and the image detected by the optical sensor array. Light from the display itself may add noise or ambiguity to the image detected by the optical sensor array, if the optical sensor array detects visible light in the same wavelengths as that emitted by the display.
In a shadow mode of operation, the sensor array may sense one or more objects such as a finger on or above the display screen by detecting the shadow of the ambient light cast by the object. The image from the sensor array is then processed to obtain the X,Y coordinates of the object(s). A sensor array used in shadow detection may require a very high dynamic range in order to detect shadows in ambient lighting the illuminance of which can vary over many orders of magnitude. If the ambient lighting is too dark, there is no shadow, and the method fails completely. Moreover, shadow detection is unable to detect patterns, designs, and other details on the object surface that is in shadow.
In a reflective mode of operation, a controlled light source is used to illuminate one or more objects such as a finger on or above the display screen. The backlight is a controlled light source, and by turning all pixels on in a color LCD, a uniform white light is transmitted through the display. The reflection of this light from the object(s) may be detected by the optical sensor array and processed. However, if the LCD is displaying a black image, then the backlight is not illuminating anything in the region above the image and any objects in that region will not be detected. An arbitrary image displayed on the LCD will affect how much of the backlight is transmitted through the LCD and therefore the illumination of objects on or above the display screen. Consequently, in reflective mode, an arbitrary image on the display screen may interfere with the detection of objects on or above the display screen.